1. Field of the Invention
This invention relates to a negative hydrogen or deuterium ion source, which negative ions play an important role in generation of large-capacity high-temperature plasma of a nuclear fusion reactor. More particularly, the invention relates to a source of negative hydrogen or deuterium ions which is adapted to produce a high-quality negative ion beam with a high electric current density by using a solid-state element including a semiconductor portion.
2. Description of the Prior Art
The neutral particle beam injection (NBI) is important in the research of nuclear fusion. Recent research in Japan and other countries has proved that, when a magnetic field holds a plasma, the neutral particle beam injection is an effective method of heating the plasma to a high temperature, or an effective method of generating such a hot plasma, without largely disturbing the holding ability of the magnetic field.
A conventional method of producing energetic neutral particle beams for injection into a plasma comprises steps of accelerating protons or deuterons so as to form beams, and neutralizing the proton or deuteron beams by the charge exchange collision in the gaseous target. Such conventional method of producing energetic neutral particle beams has shortcomings in that its neutralizing efficiency decreases with the increase of the energy of the protons or deuterons, so that, in the case of the deuteron energy in excess of 200 keV as required for actual thermonuclear fusion reactors, (1) the neutralizing efficiency becomes lower than 20%, (2) a very large evacuating capacity is required, and (3) the durability of the source to generate protons or deuterons becomes low.
To overcome the above-mentioned shortcomings of the conventional method of producing the energetic neutral particles by using protons or deuterons, a method of using negative ions is contemplated, in which negative ions are produced and accelerated and then neutralized by detachment of an excess electron. If it is intended to produce negative hydrogen ions H.sup.- or negative deuterium ions D.sup.- from hydrogen molecules H.sub.2 or deuterium molecules D.sub.2 through a conventional process, the yield of negative ion is not so large because the electron affinity of such molecules is as low as 0.7542 eV, and no effective method has been developed yet for producing such negative ions with a sufficiently high yield for practical application. More particularly, researchers are now studying two experimental negative ion producing methods; namely, an experimental method comprising steps of applying cesium (Cs), which has a low ionization potential, onto the metallic surface, creating protons or deuterons by gaseous discharge, and producing negative hydrogen or deuterium ions by using the two-electron capture reaction of the thus created protons or deuterons on the metallic surface; and another experimental method which produces negative hydrogen ions H.sup.- or negative deuterium ions D.sup.- by dissociating hydrogen molecules H.sub.2 or deuterium molecules D.sub.2 through electron impact or photoexcitation. The experimental methods of the prior art to produce negative ions have shortcomings in that the yield of negative ion is low, and that a comparatively large quantity of impurities such as cesium (Cs), electrons, and neutral gas are emitted from the negative ion source. Accordingly, there are a number of problems to be solved before establishing a practicable method of producing high-quality negative ion beam whose output is 10 to 50 A and large enough for carrying out the studies of nuclear fusion; such as elimination of impurities from the negative ion beam to be injected into the plasma, treatment of undesired electrons whose electric charge is of the same polarity as that of the desired negative ions, evacuation of a large amount of gas, and simplification of the intricate structure of the negative ion source.